The present invention relates to a system for reducing a noise of a signal transmission medium and particularly to a noise reduction system for an audio signal recording and reproducing apparatus such as a tape recorder, and more particularly to a noise reduction system useful for a recording medium with narrow frequency band, high noise level and low saturation level, such as a cassette tape recorder.
Known is a noise reduction system of a compander system with excellent effect to improve signal-to-noise ratio (S/N) manufactured by dbx company of U.S.A. Details of this compander system are treated in U.S. Pat. Nos. 3,681,618, 3,789,143 and 3,714,462. A brief and only related description will be given here.
Generally, when a noise reduction system using a compander system having a wide control range and excellent effect to improve signal-to-noise ratio is used for a recording medium with narrow dynamic range device such as a cassette recording/reproducing device recorder, it is noted the following two phenomena.
1. Noises are scarcely heard in low- and middle-levels because of its excellent effect to improve signal-to-noise ratio. Accordingly, a modulation noise generated in the case of a high level signal is rather noticeable.
2. The noise level of the recording medium varies in accordance with the expansion operation at the reproducing period. Thus, when a recording medium with a noise level of -40 dB is used in the above-mentioned noise reduction system of dbx corporation, the improvement factor of S/N in this system is expressed -178 x (input signal level (dB)). For example, the improvement factor of S/N for the input signal -100 dB is 50 dB and the noise level is -90 dB, thus eliminating most of noise. For 0 dB input signal, the S/N improvement factor is 0 dB and the noise level is -40 dB, and noise level also is modulated with the level of a source.
Since the noise level modulation due to the above two phenomena is remarkable, when the recording or reproducing of a sound source with relatively simple harmonic constitution having small masking effect such as piano solo, the effects of the noise level modulation are not negligible in practical use.
For reducing the noise level modulation effects discussed above with regards to the second phenomenon, it is conceivable to additionally employ emphasis and de-emphasis circuits used to reduce the high frequency noise of an intermediate transmission (recording) medium such as a magnetic tape, and further to use a weighting operation is used in expanding and compressing the high-frequency range in order not to reduce the peak margin to the saturation level of the recording medium.
To the recording medium with narrow frequency band, high noise level and low saturation level such as cassette tape recorders, when such an improved system is applied, the noise level modulation occurs and the high frequency range of signal is not reproduced so that the reproduced sound is not clear. Further, since the level of the high frequency is high, there appears the noise level variation of the above-described first phenomenon. When an encoded signal formed by emphasizing a high level signal in the high-frequency range is recorded, deterioration of the high frequency characteristic and the increase of the noise level modulation are particularly remarkable.